1. Field of the Invention
The present invention relates to avoiding the deleterious polycondensation disrupting effects of the impurities in an impure N-dichlorophosphoryltrichlorophosphazene of the formula: ##STR1##
2. Description of the Prior Art
In its monomer form, N-(dichlorophosphoryl)trichlorophosphazene (abbreviated as P.sub.2 NOCl.sub.5) is used for the preparation of poly(dichlorophosphazene) by a process described, for example, in French Patents 2,466,435 and 2,571,710, in accordance with the following general scheme: EQU n Cl.sub.2 P(O)NPCl.sub.3 .fwdarw.Cl.sub.2 P(O)(N.dbd.PCl.sub.2)n Cl+(n-1)POCl.sub.3.
This preparation of poly(dichlorophosphazene) by polycondensation requires a P.sub.2 NOCl.sub.5 of very high purity. Depending on the purity, it is possible to produce a poly(dichlorophosphazene) of more or less high molecular weight. In the event that the purity is wholly inadequate, a completely crosslinked polymer is obtained.
The impurities in P.sub.2 NOCl.sub.5 are responsible for the following two adverse phenomena:
(i) they terminate the polymer chain growth at a certain level of molecular weight;
(ii) they induce reactions which interfere with the polycondensation and promote branching and interchain bridging to ultimately provide a crosslinked polymer that is virtually useless.
The quality of the P.sub.2 NOCl.sub.5 can thus be evaluated in terms of its behavior, as a result of which very great differences in reactivity with respect to the polycondensation thereof, as a function of the conditions for the preparation of the P.sub.2 NOCl.sub.5, are observed. Thus, the conditions indicated, for example, in French Patents 2,612,169, 2,612,171 and 2,606,396, influence the behavior of the P.sub.2 NOCl.sub.5 during the polycondensation thereof. Accidental contamination can also interfere with such polycondensation and it has been observed, for example, that trace amounts of moisture can limit the molecular weight or cause crosslinking of the polycondensation product.